If there is an air bubble in a cooling system it can cause the system to not work at all and overheat!
If the bubble gets inside the pump the impeller will just spin in the air, no water will move. If it gets inside the block or rad then the water will not transfer heat, the air will block it. Very dangerous.
You realise I am talking about a large bubble, larger than the diameter of the tubes. The tiny bubbles are not good, but are not dangerous like the bigger bubbles.
maybe, maybe not. One of the things I have heard of is people who connect the input to the block or radiator above the output. This makes the water flow through without making contact with all sides of the tube. You would get cooling, but not as much as there would be if there was more contact.